It is proposed to study the role of humoral antibody in the protection and destruction of allografts and xenografts of normal tissues and organs in mice. Protective effects of antibodies - immunological enhancement - are studied in special strains of mice which permit the use of grafts that contain only one H-2 antigen that is absent in host tissues. In this way it is possible to specifically block the total host response. Destructive effects of antibodies are studied in a system in which rat skin or kidneys are grafted to mice whose immune responses are suppressed by surgical removal of the thymus and treatment with heterologous antimouse lymphocyte serum. These two models can be used to study the contrasting effects of antiserum and the mechanisms which lead to prolonged or curtailed survival of grafts. Attempts will be made to destroy rat xenografts of skin by adoptive transfer of syngeneic, allogeneic and xenogeneic lymphoid cells.